1. Field of the Invention
The present invention relates to a plant for purifying contaminated air in which the impurities are transferred from the air to a liquid in a spray booth through which the contaminated air is caused to pass in intimate contact with the liquid which also flows through the same booth, the liquid being collected in a container from which it is recirculated to the spray booth.
2. Description of the Prior Art
The utilization of a spray booth is well known, for instance, in spray painting, the object to be painted is placed so that the excess paint will accompany an air stream through the booth. In the booth, the excess paint is brought into intimate contact with a liquid, wherein pigment and solvent contained in the paint are absorbed by said liquid, thus purifying the remaining air.
The liquid flowing through the spray booth is usually water which circulates in a closed system. Since the hydrocarbon-based solvents present in paints have limited solubilities in water, a saturation limit will very soon be reached. As a result thereof, the hydrocarbon concentration in the air which has passed through the spray booth will be unacceptably high. It has long been recognized that the exposure of people to this atmosphere in a room where spray painting is in progress may represent a health hazard.
In the United States for instance, hydrocarbon emission regulations have been imposed by federal, state and local governments. Inter alia, these emission regulations require a hydrocarbon emission reduction of approximately 80% in painting with specific laquer topcoat paints. This goal may be reached by combining different measures to obtain the desired effect. Such measures include the use of other less harmful paints and the introduction of changes in the painting process. The changes include utilizing electrostatic painting and adding hydrocarbon emission abatement systems after the painting station.
Large and expensive abatement systems have been installed to purify the solvent-contaminated air coming out from the spray booth. These systems work by passing the air through a purifying plant containing activated carbon or some kind of scrubber in which the solvents from the spray booth are washed out.
According to another type of air purification, the solvent-contaminated air is passed through an incineration oven or some other kind of heating arrangement where the poisonous airborne hydrocarbon compounds are burnt.
In the prior art there are examples of plants for purifying air, such as the one described in the German Patent DE-B No. 25 47 675. The plant in this patent employs a scrubber where the circulating scrubbing liquid is exposed to a biological purification stage. However, using such a plant for purifying the exhaust air coming out from a scrubber, would be extremely costly, since the plant must be dimensioned for the greater quantities of air which flow through a spray booth. In practice therefore, the problem has not been solved in this way due to the high costs involved.
Consequently, it has hitherto not been possible to solve both the economical and the practical problems which are associated with the emission of hydrocarbons contained in spray booth air. Moreover, those systems which have presented practical solutions to the problem, have not met the minimum emission regulations which the authorities have imposed and which already need be met by the middle of this decade.
Accordingly, there still remains the need for a system to purify contaminated air flowing in intimate contact with a liquid, which system meets minimum emission regulations while being practical and economically feasible.